1. Field of the Invention
The present invention relates to a pressure detector operable as pressure sensor or pressure switch.
2. Description of the Prior Art
FIG. 11 and FIG. 12 represent a conductive rubber switch working as the prior art pressure detector disclosed, for example, in Japanese Utility Model Laid-Open No. 39523/1985. In the drawings, 1 denotes a conductive rubber sheet or comb-line electrode, 2 denotes another conductive sheet with at least three insulating projections 3 provided thereon at suitable intervals, and both the conductive rubber sheets 1, 2 are built up together through the insulating projections 3 so as not to come in contact with each other under non-pressurized state. A reference numeral 4 denotes a flexible sheet consisting of rubber or synthetic resin with at least one tapered projection 5 provided thereon at a position not to overlap with the insulating projections 3, which is placed over the conductive rubber sheet 2. Reference numerals 6, 7 denote electrodes, 8 denotes a power source, and 9 denotes an ammeter.
In a compound conductive rubber switch 10 in a layer-built structure constructed as above, when pressure is not applied on the flexible sheet 4, a resistance between the isolated electrodes 6, 7 is infinite, and thus no current flows any. If pressure is applied on the flexible sheet 4, then the conductive rubber sheet 2 is curved to touch the conductive rubber sheet 1 as directed by a chain line, and a series resistance circuit is formed between the electrodes 6, 7 to allow the current to flow therethrough. Since the current value changes according to the resistance which resistance changes in accordance with the pressure applied to the flexible sheet 4, the magnitude of the applied pressure is obtainable from measuring the current.
This prior art device relies on deformation of one sheet in order to achieve contact with the second sheet. The two sheets are separated by, and thus insulated by, the projections 3 when in the non-pressurized state. Among the disadvantages with such an arrangement are: (a) low pressure cannot be detected, (b) pressure detection involves a heavy dispersion and where the pressurized condition continues for a long period of time a permanent strain of the conductive sheets may result, (c) since a change in resistance value to the change in pressure is not linear, a correction by arithmetic circuit or the like is necessary, accordingly while the prior art detector is available for use on rough-and-ready detection of loads (for example, detection of the number of passing cars), it is no more serviceable for precise detection of loads (for example, weight measurement of objects).